3D calculations of large aperture opposite field septum magnet

1
3D calculations of large aperture opposite field
septum magnet

• K. Fan, I. Sakai, Y. Arakaki, M. Tomizawa
• KEK, Japan

2
Outline

• Introduction
• 3d Calculations
• Comparison with measurement
• Summary

3
Introduction

• High intensity proton synchrotron needs large
  aperture magnets
• space charge effect causes large beam size
• Inj./Extr. System
• Septum magnet system is divided into several
  groups. (beam separate effectively )
• Each septa becomes shorter (gap/length is larger)
• End field becomes severe (nonlinear field affects
  beam quality)
• 3D calculation is important
• Estimate the detail field distribution.

4
J-PARC 50GeV injection Septum magnets

• Injection septum magnet system
• High field septum, normal structure, 1.36T
• Medium field septum, opposite field structure,
  0.6T
• The septa system is simple (benefit from opposite
  septa)

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Opposite field septum magnet system

• The system consists of 3 magnets
• The total length of sub-magnet is the same as
  septum magnet
• Sub-magnets increase By for inj. beam(same
  polarity)
• Sub-magnets cancel By field for circulating beam
• Force free of septum
• Thin, support easily, pulse mode operation, heat
  problem

6
3D model of the septum magnet system
Gap adjusting
Gap adjusting

• Septum magnet is fixed (septum conductor in
  vacuum chamber)
• Integral By of sub-magnets is larger (effective
  length long)
• increase the sub-magnet gap (important feature)
• If the integral By of sub-magnet is lower, how to
  do?
• Cut sub-magnet / increase septa gap ?

7
Septum conductor position affects field
distribution
Chamber

• In real case, the magnet field near septum not
  uniformity
• Gap between core and septum conductor.

8
Simplify the model of septum
Real shearing
gap
simplified
Air, reduced

• Simplify the structure
• Make the volume mesh easy
• Without much precise loss

9
Real structure of the conductor
High current density

• Conductor shape, 70 8-node bricks
• Current 60kA

10
Longitudinal magnetic field with different gap

• By distribution with different Sub-magnet gap.
• Sub-magnet field decreases, septum doesnt change
• By inside septa not uniform Longitudinally
• Sub-magnet not symmetry
• Reduced potential only near conductor
• Background volume smaller, large mesh size.
  (100,000nodes1day)

11
Integral field with different sub-magnet gap

• Increase sub-magnet gap
• Integral field at injection side decrease (5cm
  from center)
• If the gap9.6mm, Bcir/ Binj0

12
Longitudinal magnetic field distribution

• Fine mesh at end of magnet(end field)
• By in septum magnet not uniform longitudinally
• Sub-magnet affect
• Background volume small?

13
Bx, Bz Longitudinal distribution

• Integral Bx and Bz at the center of circulating
  beam not zero
• Complicated conductor.
• BxL-108Gs.cm, BzL13.6Gs.cm
• ByL1.02106Gs.cm

14
Transverse magnetic field distribution

• By distribution _at_ sub-magnet uniform
• By near septum conductor not uniform
• Gap between core and septum conductor

15
By distribution of end field

• The end field changes much
• High vertical gap, Sub-magnet (12/35) , septum
  magnet (12/70)

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3D By distribution of end field
Sub-magnet1 - septa
Sub-magnet2- septa

• Three dimensional By distribution at both end of
  septa

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3D By distribution of the whole magnet system

• Three dimensional By distribution of the whole
  system

18
Transverse distribution of integral magnetic field

• Integral By distribution at both sides not bad
• The BL close to zero at the center of circulating
  beam
• Nonlinear field, quadrupole

19
Comparison with initial measurement

• Long search coil.
• Integral field as reference, in case of
  non-stable power supply
• Short search coil
• 2 types coils
• dB/dt is small, N300turns

20
Comparison of longitudinal field distribution

• Normalized By longitudinal distribution
• Measured data supports the calculation
• Error comes from search coil position measurement

21
Transverse distribution

• Inside septum, the measured data agrees the
  calculation
• End field does not agree
• Longitudinal distribution changes much, search
  coil size

22
Summary

• The 3D calculations examine the design of
  opposite field septum magnet.
• Initial measurement data supports the
  calculation.
• The next work
• Exact measurement
• Beam optics calculations
• Eddy current calculation